Holographic mirrors which have the property of reflecting only a band of optical wavelengths have been widely discussed as a means for protection against various laser hazards. Such laser hazards include both those created in the context of military situations on the battlefield or in the air as well as in commercial and industrial facilities. These hazards create a chance of retinal damage as a result of a beam of high intensity laser light reaching the eye of an individual.
In its most general case, a planar mirror formed as a volume phase hologram in an optically sensitive emulsion will, in the simplest case, have the characteristic of reflecting only incident light in a relatively narrow range of wavelengths centered on the wavelength at which the mirror was recorded, provided that light is incident at the same angle at which the mirror was formed. In the simplest case of a planar mirror, this would be light incident at a right angle.
Protective systems of this sort suffer from a number of inadequacies. For example, emulsions presently available are usually sensitive at only one particular laser frequency and other wavelengths at which lasers output light will minimally or substantially not at all expose the photosensitive emulsion. Another problem involves the fact that, in accordance with Bragg's Law, the wavelength range of the notch will vary as a function of the incident angle of incoming radiation. The problem is yet further complicated if the object to be protected moves over a range of particular points, (such as the eye does), and if the shape of the protective member, such as a helmet visor, is curved. In addition, given the fact that such situations define a volume to be protected substantially greater than a point, and the further additional complication of having to provide a reflective notch at a variety of angles for a particular laser wavelength without substantial coloring of the outside world and/or reducing visual transmission to see through the filter, substantial complicating factors exist.
An approach is to widen the bandwidth of the notch filter, thus making it effective over a wide range of angles. This technique has the distinct disadvantage of seriously compromising transmittance of the protective filter at wavelengths other than those at which reflection is desired, thus darkening the view of the outside world and compromising visual acuity in what is often a high criticality situation, as would be the case, for example, in military applications. Widening the notch will also have the undesirable effect of significantly coloring the perception of the outside world.
Still another approach to the problem is the fabrication of the notch filter on a substrate of a given configuration, followed by transfer of the exposed hologram to a substrate of a different configuration (U.S. Pat. No. 4,802,719: Infrared Laser Shield). Also, in my earlier U.S. Pat. No. 4,786,125 entitled Ocular Protection Apparatus, in which this technique is used to minimize the angles of possible laser-light incidence with respect to normals to an optical surface protecting the eye of an individual.
Still yet another approach is the use of a multiple element holographic mirror. Typically, in accordance with this type of solution, the angular characteristics of each of the elements is made different from the other. For example, one of the layers may be designed to protect one of the eyes of an individual while the other layer may be designed to protect the individual's other eye, with both layers disposed over and completely covering a transparent helmet visor.
In particular, one approach that has been proposed is the use of two holograms each of which is designed to protect one of the eyes of an individual who is wearing a visor which incorporates the two holograms. Generally, each of the holographic protective layers will comprise a volume phase hologram in which the planes in the volume of the volume phase hologram form concentric spheres centering on one of the eyes to be protected while the concentric spheres in the other holographic film center on the other eye to be protected. While such an approach would appear to present an ideal solution to the problem, several inadequacies will present themselves.
In particular, as noted above, a visor constructed in accordance with this technique consists of two slanted holograms with a spherical configuration, each having the center of curvature in the safety zone. Moreover, the normals for each hologram are designed to pass through the centers of curvature of the spherical mirrors which form the hologram. However, the slanted configuration of this design creates, by the intersection of planes of diffraction with the surface, an unevenly spaced surface grating. This uneven surface grating produces diffractive ghost images.
In addition to diffracted ghost images produced by the surface grating, both ghosts and flare might be created by secondary holograms recorded in the sensitive holographic emulsion by multiple internal reflections which occur during construction of the interference pattern during exposure of the hologram. This results in multiple recordings and even if anti-reflection coatings are used, such multiple recordings will produce serious ghost images and so-called flare where the tendency is to separate colors of light but not by such a great angular deviation as to generate separate ghost images. Generally, multiple recordings produce ghosts, not by surface diffraction, but by the super imposition of planes of diffraction which are closely recorded in space.
If we consider these problems in the context of, for example, an aviation cockpit, where through the windshield visual acuity is critical, it is obvious that the diffractive ghosts and flare should be eliminated or greatly reduced. Also, the construction of the required fast spherical mirrors on the visor is an extremely difficult task. This is particularly so with regard to obtaining repeatability in coating double curvature substrates (visors) and by the optics required to produce the hologram which can require a close to 180 degree cone of illumination.